Pyrometers are known in the art which contain a single-element detector to which the thermal radiation emitted by an object is delivered by means of an optical system. However, in this case the problem occurs that the characteristic thermal radiation of the optical system also enters into the output signal of the detector. In order to reduce the characteristic radiation of the imaging optical system, the use of high-grade, slightly absorbent materials is proposed in refractive optical systems and the use of very reflective surfaces is proposed in reflective optical systems.
Another possibility known in the art resides in the electronic compensation of the signal when the temperature of the optical system is known. For this purpose, in a pyrometer a second detector is provided which is covered by a reference element disposed immediately in front of it, so that the reference radiation emanating therefrom falls onto the second detector. By means of a correspondingly arranged temperature sensor the temperature can be determined in the measuring device and can then be taken into account in the processing of the output signals of the two detectors in a corresponding device.
However, this electronic compensation is not suitable in the case of an irregular temperature distribution within the optical system, such as occurs for example in the case of abrupt changes in the ambient temperature. Such temperature changes can occur for example due to the device being picked up in the hand.